1. Field of the Invention
The present invention relates to a diagnostic apparatus for an internal combustion engine.
2. Description of the Related Art
Many internal combustion engines, such as vehicle engines and the like, are equipped with a well known diagnostic apparatus that determine normality/abnormality of the engine and locates an abnormality if any is detected. Such a diagnostic apparatus has a self-diagnostic function, diagnosing an abnormality based on output signals of various sensors that detect operational conditions of the engine. The apparatus constantly diagnoses whether there is a failure, and performs an operation for warning a driver and/or a fail-safe operation, etc.
An example of the aforementioned diagnostic apparatus described in Japanese Patent Application Laid-Open No. HEI 8-75612 detects the occurrence of misfire based on a change in the rotating speed of an output shaft of an internal combustion engine during an explosion stroke of the engine. Based on the frequency of detection of misfire, the apparatus determines whether the engine has an abnormality. Upon determining that the engine has an abnormality, the diagnostic apparatus immediately turns on a warning light to inform a driver of the occurrence of the abnormality.
Although it is important to notify a driver of any detection of abnormality, it is not always the case that the detected abnormality needs urgent treatment; nor is the detected abnormality necessarily permanent. That is, detected abnormalities are often transitory phenomena or phenomena that occur only under specific conditions. Therefore, the abnormality may simply dissolve when the engine is restarted. The incidence of misfire in particular, varies considerably depending on external factors, such as ambient air temperature, pressure conditions, and the like. Thus, even if there is no problem with the internal combustion engine, a misfire abnormality may be indicated depending on the operational condition of the engine, the operation environment, and the like.
Therefore, if a warning is given every time an abnormality is determined, as in the above-described apparatus, the incidence of inappropriate and/or unnecessary warnings will increase so that a driver may be annoyed or may become doubtful. The incidence of a warning can be reduced by elevating a criterion for determining an abnormality. However, an elevated criterion will also reduce the precision of the detection of abnormalities of which the driver needs to be notified.